I mainly shoot sports with my D4 body, a 80-200 AFD and 200-400 f4 VRII. This weekend I went for some birds shooting which I really enjoyed but we were usually far from our main subjects. I realized that I would need a 600mm f4 if I want to get more serious about it based on the location I would usually go to take pictures.

I already have a 1.4 TC and it does a decent job with the 200-400. But I would need more, would often be 600 prime + 1.4x once in a while.

I'm not sure if that would be a potential solution that would cost less money and would be a backup body which I currently don't have: a D800. Since it has over twice the resolution of a D4, could that be a good alternative instead of going to a 600 f4 prime ? Would be much cheaper and smaller that's for sure, that way I could carry the two bodies and my 200-400.

Please let me know if any had a chance to do this kind of comparison (higher resolution vs longer lenses)